System and method for microwave ranging to a target in presence of clutter and multi-path effects

ABSTRACT

A system for measuring the range to an RFID tag including situations containing high clutter and multi-path signals is disclosed. The system includes an RFID reader; an RFID tag; and a coordinated pulse compression radar system. In the system the RFID reader causes the tag to respond to received signals in a first backscatter state at a first time and a second backscatter state at a second time. The pulse compression radar system transmits short pulses coordinated by the backscatter state of the RFID tag and the system creates a differential signal comprised of the differences between radar signals obtained during the first and second states of the tag to obtain an uncorrupted measure of a round trip time of flight of said radar pulses between the pulse radar system and the RFID tag.

CROSS-REFERENCE TO RELATED APPLICATIONS

This utility application claims the benefit under 35 U.S.C. §119(e) ofProvisional Application Ser. No. 61/355,824, filed on Jun. 17, 2010,entitled System and Method for Microwave Ranging to a Target in Presenceof Clutter and Multi-path Effects, and also claims benefit under 35U.S.C. §120 as a continuation-in-part of Utility application Ser. No.13/095,296 filed on Apr. 27, 2011, also entitled System and Method forMicrowave Ranging to a Target in Presence of Clutter and Multi-pathEffects. The entire disclosures of these applications are incorporatedby reference herein.

FIELD OF INVENTION

This invention relates to the field of electromagnetic distancemeasurement and, in particular, distance measurement to an RFID device.

BACKGROUND OF THE INVENTION

A radar system may measure the distance to a target through measurementof the round-trip time-of-flight of the radar signal to a target andreturn. The one-way distance, d, to a target is computed from theequation 2d=ct where c is the velocity of light and where t is the timebetween transmitted signals and received signals reflected from atarget. Radar technology is well developed. However, an individualtarget may be difficult to isolate if there are many reflectors in thefield of the radar system. Also, the distance calculation may becorrupted by multipath effects and limited bandwidth of the transmittedpulse. Multipath effects may be mitigated by using the time of the firstreturn signal from a target. However, this technique is fraught withproblems if the reflected signal from the desired target is hidden bysignals reflected from other objects in the field of the radar system.

RFID systems are well known in the art and are used to monitor objectsand places by attaching tags to objects and places to be monitored.These objects may be large and in the presence of many other reflectingobjects. RF signals from a tag may be hidden by noise and larger signalsreflected from other reflecting objects. Backscatter RFID readerstransmit CW signals while acquiring data from tags, and thus lack thecapability of ranging using measurement of time-of-flight methods. RFIDtags used in modulated backscatter RFID systems are often referred to as‘passive’ (without an internal source of power) or ‘semi-passive’ (withan internal source of power) since modulated backscatter tags do notgenerate radio signals and only reflect radio signals. RFID tags mayalso send data to a RFID reader by generating and sending radio signals.These types of RFID tags are often referred to as ‘active’ tags sincethey generate radio signals and contain an internal source of power. Thephase of the backscattered signals from a modulated backscatter tag canbe used to calculate the distance to a tag in the presence of otherreflecting objects, as disclosed in U.S. patent application Ser. No.12/840,587, titled SYSTEM AND METHOD FOR MEASUREMENT OF DISTANCE TO ATAG BY A MODULATED BACKSCATTER RFID READER, but accuracy may be degradedin a highly reflecting environment due to multipath effects. Many typesof RFID systems use modulation signals with frequencies on the order ofa megahertz or less and often shape waveforms to comply with radioregulations. These modulation waveforms lack nanosecond precision neededto use time-of-flight methods to measure distance to a resolution on theorder of a meter or less between tags and readers.

RFID systems using time-of-flight methods to determine object locationmay be found in the art but these types of systems are expensive,require careful installation, use expensive tags and require precisepositioning of the system components. Signal strength methods todetermine tag location may also be found in the art, but these types ofsystems lack accuracy and precision.

Many tens of millions of RFID tags are presently in use andinstallations would benefit if the distance to these tags could bemeasured accurately in a complex radio environment.

Modulated Backscatter RFID System of the Prior Art

A modulated backscatter RFID tag transfers data from its memory to aremote reader by modulating the backscatter cross section of the tagantenna in a coded fashion, changing at a minimum from one reflectingstate to another reflecting state (or between several reflecting states)in a time-wise fashion, thus coding the tag data on the time-varyingbackscatter cross section of the tag. A continuous wave (CW) radiosignal is transmitted toward a tag by a reader. The tag modulates thereflected signal sent back to the reader thus producing a time-varyingsignal encoded with data from the tag. The reader then receives anddecodes the modulated signal from the tag to extract the informationsent by the tag. The decoding process recovers the timing of the changesin modulation states of the tag. These timings cannot be used fortime-of-flight calculations since there is no absolute time reference toestablish a time base for calculation. Another practical problem is thatthe transitions from one modulation state to the other lack thebandwidth, precision and definition in timing required for nanosecondresolution required for ranging. For example, a resolution of 1 meter intag location requires a timing resolution of 7 nanoseconds or better.Typical RFID systems such as specified by ISO/IEC 18000-6: 2004(E) andISO/IEC 18000-6:2004/FDAM 1:2006(E) require timings, such as rise andfall times, on the order of microseconds and are thus over 1000 times toslow. The reader also decodes the states of modulation as a function oftime. The reader uses these states to recover the bit pattern, and thusdata, sent by the tag.

Pulse Radar System of the Prior Art

An example of the geometry of a conventional radar system is shown inFIG. 1. The radar system transmits a RF signal which is reflected fromthe objects in the field of the radar and are received by the radarsystem. Strong multipath signals may occur from a radio path bouncedfrom the radar system to a flat surface (ground for example), totargets, and return. A sample plot of signals is shown in FIG. 2. Tomeasure the distance to a single desired target, the correct returnsignal of the many in FIG. 2 is required.

All references cited herein are incorporated herein by reference intheir entireties.

BRIEF SUMMARY OF THE INVENTION

Objects of the invention include:

-   -   Measuring distance to RFID tags that are already in use    -   Measuring distance to a RFID tag with an accuracy of 1 meter or        better    -   Measuring distance to a RFID tag in the presence of multipath        effects    -   Measuring distance to a RFID tag within a cluttered RF        environment    -   Measuring distance to a RFID tag in close proximity to other        reflecting objects    -   Reducing bandwidth requirements of signals transmitted to        measure distance to a RFID tag while improving accuracy    -   Improving signal to noise ratio to enhance measurement accuracy        of distance to a tag while reducing bandwidth and transmit RF        power    -   Reducing or eliminating the effects of self-jamming by the        signal transmitted by a reader to read a tag while        simultaneously determining the distance to a tag    -   Enabling a wide dynamic range to resolve weak signals in the        presence of noise, interference and other strong RF signals    -   Measuring distance to a moving RFID tag avoiding averaging        signals for long periods of time    -   Shortening the minimum range distance due to self jamming of the        transmit radar signal    -   Measuring distance to a RFID tag while simultaneously in data        communication with the tag

The present invention achieves the stated objectives as well as otherswhile overcoming difficulties of the prior art. The techniques of thepresent invention can be applied to other radar systems using theprinciples described below.

In an embodiment of the invention, there is a system for measuring therange to an RFID tag including situations containing high clutter andmulti-path signals, is disclosed. The system includes an RFID reader; anRFID tag; and a coordinated pulse radar system. In the system, the RFIDreader causes the tag to respond to received signals in a firstbackscatter state at a first time and a second backscatter state at asecond time. The pulsed radar system transmits short pulses coordinatedby the backscatter state of the RFID tag and the system creates adifferential signal comprised of the differences between radar signalsobtained during the first and second states of the tag to obtain anuncorrupted measure of a round trip time of flight of the radar pulsesbetween the pulse radar system and the RFID tag.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic of a radar system of the prior art including atarget, multipath and clutter.

FIG. 2 shows a transmit and a receive signal of the radar system of FIG.1.

FIG. 3 is a schematic of a combined RFID system and radar system of thepresent invention.

FIG. 4 shows signals of the system of FIG. 3.

FIG. 5 is a series of signals of the system of FIG. 3 with theconditions of a transmit radar signal with a frequency of 1 GHz andduration of 20 ns which are processed to determine the round trip timeto a tag in the presence of a single source of clutter.

FIG. 5 a is a graph of a transmit radar signal.

FIG. 5 b is a graph of a return radar signal from a single source ofclutter while a tag is not reflecting.

FIG. 5 c is a graph of a correlation of the transmit radar signal withthe return radar signal of FIG. 5 b.

FIG. 5 d is a graph of a of a return radar signal from the single sourceof clutter and a tag that is reflecting.

FIG. 5 e is a graph of a correlation of the transmit radar signal withthe return radar signal of FIG. 5 d.

FIG. 5 f is a graph of a correlation of the transmit radar signal withthe return radar signal from the tag alone.

FIG. 6 is a graph of a series of signals of the system of FIG. 3 withthe conditions of a chirp transmit radar signal that increases frequencyof 100 MHz in 2048 ns. Radar signals are downconverted to the frequencyrange of 50 MHz to 150 MHz. The radar signals are processed to determinethe round trip time to a tag in the presence of two sources of clutterand noise.

FIG. 6 a is a graph of a transmit radar signal.

FIG. 6 b is a graph of a return radar signal from two sources of clutterwhile a tag is not reflecting.

FIG. 6 c is a graph of a correlation of the transmit radar signal withthe return radar signal of FIG. 6 b.

FIG. 6 d is a graph of a return radar signal from two sources of clutterand a tag that is reflecting.

FIG. 6 e is a graph of a correlation of the transmit radar signal withthe return radar signal of FIG. 6 d.

FIG. 6 f is a graph of a correlation of the transmit radar signal withthe return radar signal from the tag alone.

FIG. 6 g is a graph of a return radar signal from the tag alone.

FIG. 7 is a block diagram of an exemplary mono-static radar system.

FIG. 8 is a block diagram of an exemplary bi-static radar system.

FIG. 9 a is a plot of transmit signal for a vehicle at 14 feet.

FIG. 9 b is a plot of a receive signal state A for a vehicle at 14 feet.

FIG. 9 c is a plot of a receive signal state B for a vehicle at 14 feet.

FIG. 9 d is a plot of a compressed receive signal state A for a vehicleat 14 feet.

FIG. 9 e is a plot of a compressed receive signal state B for a vehicleat 14 feet.

FIG. 9 f is a plot of a differential compressed receive signal for avehicle at 14 feet.

FIG. 9 g is a plot of power of a differential compressed receive signalfor a vehicle at 14 feet.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The RFID Ranging system of the present invention is shown in FIG. 3.Clutter and multipath effects are not shown for clarity and to simplifythe explanation of the operation of the invention. The signals of thesystem of FIG. 3 are shown in FIG. 4. FIG. 4 includes signals due tomultiple targets and multipath, although the objects producing thesesignals are not shown in FIG. 3.

System Operation

The RFID reader and RFID tag shown in FIG. 3 operate in a normalfashion. The RFID reader sends several signals to the radar system asdescribed below. During times that the RFID reader is sending continuouswave (CW) signals to the tag, and the tag is simultaneously changingmodulation states, the radar system transmits RF radar signals to thetag at coordinated times and receives RF radar return signals from thetag (shown as pulses for illustration in FIG. 4). On occasion, the RFIDreader may send modulated signals to the tag to control tag operationand send data to the tag (not shown in FIG. 4). During these times, theradar system is not required to transmit signals since the tag may notbe modulating. While the RF carrier frequencies of the two systems maybe the exactly same, there may be advantages if the RF carrierfrequencies of the two systems are different from each other. The RFfrequencies of the RFID system and the radar system may be in the sameRF band or may be in different RF bands. For example, the RFID frequencymay be 915 MHz and the radar frequency may be 10 GHz. A requirement isthat the tag modulate its backscatter cross section with synchronizedtiming in the two bands.

The RFID reader decodes the information from the tag and produces aclock signal and synchronized signals indicating the state of modulationof the tag. The clock signal may be generated in the tag and recoveredby the reader, or the reader may control the clock signal. Both of thesemethods and others are compatible with the present invention. Therequirement is that the radar system know the modulation state of thetag and the times when the modulation state changes. The RFID readerrecovers the modulation state to decode the data sent by the tag. TheRFID reader sends the clock and modulation state signals to the radarsystem to be used for timing. An example of a “Clock Signal” of 160 kHzis shown in FIG. 4. The reader generates the TAG MODULATION STATE whichis shown in FIG. 4. The reader produces a signal to indicate when thetag is in “STATE A” and may produce an additional signal to indicatewhen the tag is in “STATE B”. These signals are shown in FIG. 4. Thereader sends these signals to the radar and/or processing sections ofthe system as shown in FIG. 3. The signals sent from the RFID Reader tothe radar system may be of normal bandwidth and jitter for signals inthe RFID system. High bandwidth and low jitter is only required for theradar ranging signals shown in FIG. 4.

The radar system shown in FIG. 3 transmits a RF signal to the tag duringa convenient time when the tag is in modulation State A (example of thetime labeled tA1, and receives a return signal VA (labeled with thecircled numeral 1, FIG. 4). The signal VA is delayed or stored for laterprocessing. Subsequently, the radar system transmits a signal to the tagduring a convenient time when the tag is in modulation State B (exampleof the time labeled tB1, and receives a return signal VB (labeled withthe circled numeral 2, FIG. 4). A difference signal is produced bysubtracting the stored and/or delayed signal VA from the signal VB (orvice versa) using a common time reference such as time measured from thetime of the transmit radar signal in each case. The output signal isV3=VB−VA (shown as “DIFFERENCE RECEIVED SIGNAL” in FIG. 4). The timingsof signals VB and VA are referenced to the time of the beginning of thetransmission of each individual RF signal transmitted by the radar.Thus, in the example, the timing of signal VA is delayed. The delay maybe accomplished, for example, by sampling the signal and storing in ashift register, random access memory, or other methods such as a delayline. The subtraction of stored signals can be done by a microprocessor,a digital signal processor, or other computing device at normalcomputing speeds or by an analog subtraction. Accurate, low jitter andrepeatable timing is required referenced to the timing of the particulartransmit radar signal for each received radar return signal from thetag. Alternatively, a delay line such as a SAW delay line could be usedto delay the radar return signal from a tag in one modulation state to atime when the modulation state of the tag changes to another state fordirect analog subtraction. This option requires nanosecond accuracy indelay and launch of the second transmit radar signal (B) referenced fromthe time the first transmitted radar signal (A).

The shortest time between the start of signal VA and the end of signalVB may be about 8 microseconds for the type of RFD) systems referencedabove. For example, the first transmit radar signal (A) may start 4microseconds before a transition between states and be 2 microsecondslong. The second transmit radar signal (B) may start 2 microsecondsafter the transition between states and be 2 microseconds long,resulting in a total time of 8 microseconds between the beginning andend of a set of the radar signals. If a tag is traveling at 200kilometers per hour, the tag will move 0.44 mm during this time,resulting in insignificant error or jitter in measurement of distancecompared to the desired precision. The doppler shift of a 1 GHz radarsignal for an object speed of 200 kilometers per hour is 370 Hz, or 0.37ppm. Thus, the distance to tags moving at high rates of speed can bemeasured with the methods of the present invention.

The only change between VA and VB of the targets in the field of theradar system is the modulation state of the tag. V3 contains only asignal (or signals if there is multipath from the tag) arising from thedifference in modulation state of the tag. The time of the first signalobserved in the difference signal V3 is due to the direct (shortest)distance between the radar system and the tag. Latter signals are due tomultipath, as shown in FIG. 4. In this example, the round trip traveltime of the radar signal to the tag and return is 65 nanoseconds, whichcorresponds to a one-way distance of 9.8 meters between the radar systemand the tag. The system may be calibrated to remove delays due toconstant distances such as the lengths of coaxial cables and the like inthe radar system. The difference signals may be accumulated, averaged,or otherwise processed to improve signal to noise and resolution oftiming. FIG. 4 shows radar signals as pulses for clarity of illustrationbut various types of signals may be used. The choice of radar signals isdiscussed below.

Thus, the distance to an individual tag can be found in the presence ofclutter and multipath. The RFID reader may read the tag identificationnumber or other data stored in the tag simultaneously as the radarsystem measures the distance to the tag.

Distance to an individual tag may be determined in the presence ofmultiple tags in the field by several methods. If the system uses tagsthat can be controlled, then all tags may be commanded to be silentexcept for the desired tag. If several tags are in the field and cannotbe commanded on or off, then the difference signal V3 may be averaged.Only the modulation of the desired tag will be in synchronous with theradar signals. The radar return signals from other than the desired tagwill diminish with averaging since these other signals occur at randomtimes.

The radar system may operate with higher bandwidth and lower power thanthe RFID system to provide the required bandwidth needed for timing andalso to comply with regulatory requirements.

The invention may be applied to other applications and implementations.For example, a target switching between modulation states at a constantrate may be used in place of a tag. A tag may use a second channel forradar ranging at a much different frequency than the RFID system. Forexample, the RFID system may operate at 915 MHz and the radar system at5.8 GHz.

Detailed Description of Implementations

The present invention may be implemented using several approaches toimprove performance of the basic ranging system outlined above.

Since a homodyne-type receiver is not required by the radar system, theradar return signals may be down converted to a convenient IF frequencyfor measurement and processing. The effects of self jamming and solutionfor a modulated backscatter RFID system using a quadrature homodynereceiver are well known in the art and consist of downconverting tobaseband and filtering which achieves RF frequency stability andaccuracy between transmitted and received signals economically withnormal homodyne receiver designs to filter unwanted signals. However,the radar system of the present invention reduces or eliminates signalsfrom unwanted sources (clutter) with a subtraction process outlinedabove. Further, a downconverting process can preserve information of RFfrequency, modulation and timing, thus permitting reduced demands on theprocessing operations by operating at lower frequencies. (Indeed, thetechniques of the present invention may be applied to a new type ofmodulated backscatter RFID reader with improved performance. If theradar pulses were transmitted several times during Modulation States Aand B, the difference technique described above may determine thetimings of the transitions between Modulation States A and B and thusenable decoding of the data sent by the tag without need for a homodynereceiver.)

The radar signals may be modulated in ways to improve performance. Themodulation may be in the form of a chirp (increasing the RF frequencyduring the signal), phase modulation using direct sequence or othertechniques, frequency modulation using direct sequence or othertechniques, amplitude modulation using direct sequence or othertechniques, or others. The modulations may be chosen to improve thepower within a radar signal, improve the signal to noise ratio, improvethe resolution of timing, reduce noise, and for other reasons. Forexample, the subtraction process outlined above may increase the noiselevel since each signal may contain uncorrelated noise. Thus, thesubtraction process will benefit from noise reduction techniques.

The fine measurement of distance to the tag (A. R. Koelle and S. W.Depp, “Doppler radar with cooperative target measures to zero velocityand senses the direction of motion”, Proc. of IEEE, V10, pp 492-493,March 1977) may be used with the present invention to produce anenhanced measurement of distance to a tag to enhance a coarsermeasurement of distance using round trip timing of radar signals.

Implementation of the present invention consists of the followingsubsystems, components, processes and steps:

1. An RFID reader acquires a tag and produces signals that indicatemodulation states of the tag and optionally the clock and/or timings ofthe changes of a modulation states of the tag.

2. A radar system transmits and receives signals coordinated with thestate and timing of modulation states of the tag.

3. The signals received by the radar system are processed to determinethe distance to the tag by:

-   -   a. Subtracting the received radar signal while the tag is        modulating in one state from the radar signal while the tag is        modulating in another state producing a difference signal. One        of the states may be when the tag is not returning a        backscattered signal. The subtraction process eliminates radar        returns uncoordinated with the timing of modulation states of        the tag. Thus only signals from the tag remain. The subtraction        process may increase the noise level.    -   b. Calculating the distance, d, to a tag by determining the        smallest time delay between the transmit and receive radar        signals of the difference signal with the formula d=ct/2 where c        is the speed of light and t is the smallest time delay. Later        signals may be due to multipath propagation and thus are        rejected. Signal correlation techniques may be used to process        the received signals to automate and/or improve performance.

Various methods may be used to implement these steps and improveperformance. Pulse compression techniques may be used to improve rangeresolution, reduce noise and improve signal to noise ratios. Pulsecompression techniques may use frequency modulation, phase modulation oramplitude modulation followed by matched filtering in the receiverprocessing. Downconversion of the signals to intermediate frequenciesmay be used. These and other processes may be performed in varioussequences. The sequence chosen may depend on selection of the importanceof other measures such as complexity, cost, ease of implementation,reduction of noise, range resolution, bandwidth, transmitted power, etc.The operations discussed above may be performed in various sequences forlinear processes and may be found useful for nonlinear processes. Forexample, the subtraction process may be done on the raw received signal,downconverted signals, before or after filtering with matched filters,in the time domain, in the frequency domain for signals processed byFast Fourier Transform (FFT), and the like. Weighing of transmitted andreceived signals may be performed to reduce the bandwidth of the signalsor side lobes in signal correlations.

Examples are presented here to illustrate several techniques and resultsusing methods and processes of the present invention.

Example 1

The following simulation illustrates the process of ranging to a tagusing the process outlined above.

The radar ranging process used in this example performs a subtraction ofprocessed signals:

-   -   Transmit radar signals coordinated with the modulation state of        the tag    -   Receive reflected radar signals for each of the modulation        states of the tag    -   Use an analog to digital converter (A/D) or equivalent to sample        the signals    -   Process each of the sampled signals using a fast Fourier        transform (FFT) to produce complex signal spectrums    -   Form the complex conjugate of the FFT of the transmit signal    -   Form products by multiplying the complex conjugate of the        transmit signal with the FFT of each received signal        individually    -   Produce a correlation for each received signal individually by        performing an inverse FFT of each product    -   Subtract the correlation obtained with the tag modulation in one        state from the correlation with the tag modulation in the other        state    -   Determine the round trip time delay from the time of the peak in        the difference of the correlations

A transmit radar signal that is unmodulated with a RF frequency of 1 GHzand a duration of 20 ns is shown in FIG. 5 a. A return signal from asingle target (clutter) is shown in FIG. 5 b, normalized to the sameamplitude as the signal in FIG. 5 a (propagation would diminish theamplitude of the return signal but this effect is important only insignal to noise considerations and not timing). For this example, it isassumed that this signal arises from a stationary target while the tagis not modulating. The state of the tag may be assigned to ModulationState A. The correlation of the return signal with the transmittedsignal is shown in FIG. 5 c and was found using the Fast FourierTransform process outlined above. The distance to this stationary targetcan be calculated from the time of the peak of the correlation, ie at atime of 60 ns. The peak of this correlation is broad and not welldefined. The correlation is greater than 50% from 50 ns to 70 ns, whichcould lead to uncertainty of accuracy if the signal contains noise.(These data are simulations using Microsoft® Excel® fast Fouriertransforms, FFT, and complex arithmetic without adjustments of scale.)

The return signal is shown in FIG. 5 d when the tag is modulating inState B in addition to the return from the stationary target. Thisexample shows the results if the delay to the tag is 10 ns greater thanthe delay to the stationary target, and if the strength of the returnsignal produced by State B is 30% of the strength of the return signalproduced by the stationary target. The correlation of the signal of FIG.5 d with the transmitted signal is shown in FIG. 5 e.

The correlation of FIG. 5 c is subtracted from the correlation of FIG. 5e and is shown in FIG. 5 f. In this example, the subtraction of signalswhen the tag is in different modulation states is done aftercorrelations have been performed using FFT techniques. Only signalsoriginating from the tag remain in FIG. 5 f. From the peak of thecorrelation, the round trip time to the tag is 70 ns and the distance tothe tag is 10.5 m. The correlation is greater than 50% from 60 ns to 80ns, which leads to uncertainty of accuracy if the signal contains noise.

Alternatively, the unprocessed signal of FIG. 5 b may be subtracted fromthe unprocessed signal of FIG. 5 d leaving only the signal due to thetag (result not shown), and distance determined by correlation or bestfit of a 20 ns pulse to the result. This direct subtraction may not beuseful if the signals are noisy since the subtraction may increase thenoise level. This is illustrated by the subtraction of unprocessed noisysignals is shown for another example in FIG. 6 g. The exampleillustrates an embodiment of the invention and shows that thesubtraction process may be performed on signals that are processedbefore the subtraction.

This radar signal of this example is not efficient with bandwidth norwill provide an accurate time delay if the signals are noisy. The −20dBc bandwidth is about 350 MHz and the correlation is above 50% within a20 ns time window. This example illustrates that the invention iscapable of ranging to a tag that is in the presence of a nearbyinterfering reflecting target and a for a tag that produces a reflectionsmaller than the nearby interfering target. Comparison of FIG. 5 c toFIG. 5 f shows that the tag return occurs 10 ns after the return fromthe fixed target, and is 0.3 times the magnitude of the fixed target or10.5 dB smaller.

Example 2

A second example follows the process of Example 1 with severalmodifications. In this example, parameters and modulations are chosen toimprove performance.

Example 2 uses the following parameters:

-   -   A transmit chirp radar signal with constant amplitude (other        than limited rise and fall times) with a frequency that        increases linearly by 100 MHz in 2048 ns    -   Rise and fall times limited to 100 ns to reduce out of band        emissions    -   Downconvert the received signals into the band from 50 MHz to        150 MHz    -   Clutter consisting of two stationary targets, the return radar        signal from the first has a round trip time delay of 60 ns and a        relative strength of 1, and the return radar signal from the        second has a round trip time delay of 70 ns and a relative        strength of 0.3    -   A radar return signal from a tag with a round trip time delay of        65 ns and a relative strength of 0.2 when modulating in Mode B        and 0.0 when modulating in Mode A    -   Uncorrelated white Gaussian noise with a relative strength of −3        dB from the signal produced by the tag modulating in Mode B is        added to each return radar signal. The correlation between noise        signals in different return radar signals is less than 0.01        (i.e. the noise in one return radar signal is uncorrelated with        the noise signal in another return radar signal).

Thus, the returned signals contain significant noise and the tagproduces a signal significantly smaller signal than the stationarytargets and has a round trip time delay within 5 ns of the othertargets. The duration of the radar signal is 410 times larger than thetime delay between the tag and interfering reflecting targets. Thisexample illustrates several features of using pulse compressiontechniques of the present invention. These features may be used togetheror in various combinations. The techniques of the example may be alteredto use coded phase and/or amplitude modulation common to CDMA (codedivision multiple access) pulse compression methods.

The transmit radar signal is shown in FIG. 6 a. The return radar signalfrom the two targets alone is shown in FIG. 6 b. The correlation of thereturn radar signal without the tag with the transmit radar signal isshown in FIG. 6 c. The return radar signal from all three targets isshown in FIG. 6 d. The correlation of the return radar signal from allthree targets is shown in FIG. 6 e. The correlation of the radar returnsignal from the tag is calculated from the subtraction of thecorrelation of FIG. 6 c from the correlation of FIG. 6 e and is shown inFIG. 6 f. For illustrative purposes, the radar return signal from thetag can be calculated from the subtraction of the signal shown in FIG. 6b from the signal shown in FIG. 6 d. This signal is shown in FIG. 6 gand was not used directly in the process outlined here. FIG. 6 gillustrates that the subtraction process increases the noise level sincethe noise levels in FIG. 6 b and FIG. 6 d are uncorrelated. The tagsignal is not discernable by casual inspection of FIG. 6 g, but thepulse compression process improves the signal to noise ratiosignificantly, and the subtraction of the correlations revels the timedelay of the tag radar return signal clearly (FIG. 6 f). The −20 dBcbandwidth of the transmitted pulse is about 108 MHz, much less thanExample 1 with much improved accuracy. (For ease of viewing, allfigures, the plots were drawn using the Excel smoothing option.) Thepeak of the correlation is at a time of 65 ns, and thus, the one waydistance to the tag is 9.75 m.

In this example, the side lobes of the correlation extend over a span ofabout 10 ns to 15 ns. Thus, targets (either individual stationary or dueto multipath) will be detected but not completely resolved in timing ifthe returns occur within about 10 ns of each other. Inspection of FIG. 6c shows that the time delay of the larger target is determined correctly(60 ns) but the time delay of the smaller target, while its effects arevisible, are not readily discernable. The correlation function is known,so curve fitting techniques may prove useful in resolving the twotargets individually from the data shown in FIG. 6 c.

This example has illustrated a preferred embodiment of the presentinvention. The self jamming effect in conventional radar is eliminatedwith the present invention since these self jamming signals appear asanother stationary target and are eliminated by the subtraction processbetween radar returns when the tag is in different modulation states.Thus the distance to a tag may be found in cases where the first returnfrom a tag occurs during the transmit signal. The radar system mayinclude separate transmit and receive antennas to reduce the effects ofthe transmit radar signal entering the received signal channel.

Simulations show that a pulse compression process outlined heresuccessfully suppresses RF signals uncoordinated with the tag modulationincluding CW signals with frequencies in the band of the radar signal,can resolve multipath signals from the tag, and parameters can be chosento optimize the system for various conditions. The examples providedhere use a chirp radar signal and matched filtering. The invention mayalso be implemented with phase modulation, Barker codes, pseudo randommodulations, and other like forms of modulation to improve performanceand accuracy.

Example 3

Methods shown in Example 1 and Example 2 illustrate the technique ofdifferential pulse compression radar to measure the distance to a tag inthe presence of clutter and multipath. Example 2 also shows the abilityof the methods to suppress amplitude noise as shown in FIG. 6 b throughFIG. 6 g. Implementation of the methods are improved by consistency intiming, frequency and amplitude and minimizing jitter of the transmittedradar pulses and subsequent data processing. An improved differentialpulse compression radar is shown in FIG. 7. The RFID Reader, Host anddata processing sections of FIG. 3 have been omitted for clarity.

Operation is performed using a minimum set of signals. First, timing isderived from a signal (from a reader or other source) indicting themodulation state of the tag being read. An example of such a signal isthe “TAG MODULATION STATE” of FIG. 4 and “Modulation State from Reader”on FIG. 7. Further referring to FIG. 7, Signal Generator 1 (Agilent MXGN5182a) provides a reference RF signal of 850 MHz in this examplealthough other suitable frequencies could be used. An arbitrary waveform generator ARB 2, (Tektronix AWG5002B) is programmed with thedesired waveform to be used in the pulse compression technique. In thisexample, the arbitrary waveform is a linear sweep from 52 MHz to 78 MHzin a period of 2 microseconds, also referred to as a ‘chirp’ signal. Thestart of the sweep is controlled by the Control and Data Acquisitionmodule 3 (LeCroy LT-354ML). The trigger to start the transmitted chirpsignal and the trigger to begin data acquisition are both synchronizedto the Tag Modulation State signal.

Control and Data Acquisition module 3 is used to coordinate timing andacquire, digitize, record and display data. Control and Data Acquisitionmodule 3 receives the Modulation State Signal of Tag 20 and generates atrigger signal to control the timing of a chirp signal produced by ARB2.

ARB 2 generates a chirp signal with a duration of 2 microseconds thatstarts at 52 MHz and increases linearly to 78 MHz. The chirp signal fromARB 2 is sent to Splitter 4 (Mini-Circuits ZAPD-1) and then to the localoscillator port of Mixer 6 (Anzac MDC-149). The parameters of the chirpcould be tailored for the modulation used by the RFID system ofinterest. Here, the values are suitable for the signals specified in ISO10374 or other RFID systems for use in the 915 MHz ISM band in theUnited States. Other choices could be made and would be within the scopeof the invention.

Signal Generator 1 produces a constant 850 MHz signal which is splitinto two signals by Splitter 4. One of the signals from Splitter 4 issent to the RF port of Mixer 6. Attenuator 9 (JFW 50R-102) is adjustedto provide the desired signal level to Mixer 6. The other part of the850 MHz signal produced by Signal Generator 1 is further split bySplitter 5 (Mini-Circuits ZAPD-1) and feeds the LO ports of both Mixer 7(Mini-Circuits ZP-2MH) and Mixer 8 (Anzac MDC-149).

Mixer 6 multiplies the signals input to its RF and LO ports producing anup-converted output signal at the IF port. The output signal contains anRF chirp signal with a duration of 2 microseconds that starts at 902 MHzand increases linearly to 928 MHz. Other unwanted signals are removed byFilter 12 (Cir-Q-TEL 21377). The desired chirp signal travels throughAttenuator 11 (JFW 50R-102), amplifier AMP 10 (Mini-Circuits ZKL-2R7),Filter 12, AMP 13 (Amplifier Research 1W1000B), Splitter 14(Mini-Circuits ZAPD-1), Isolator 15 (MA-COM 7N-195), Circulator 16(MA-COM 7N195) and is transmitted to the Tag Antenna 19 by Antenna 18.Various suitable antennas may be used depending on the desired gainpattern and other requirements for mounting and environment. The otheroutput of the Splitter 14 is sent to attenuator 17 (Agilent 8495B) andthen to the RF port of Mixer 7. The down-converted transmitted signal issent to the Control and Data Acquisition module 3 to be used in thepulse compression process.

The signal received by Tag Antenna 19 is modulated by Tag circuitry 20,backscattered, and received by Antenna 18, then sent to Mixer 8 byCirculator 16. The IF outputs of Mixer 7 and Mixer 8 are the downconverted transmitted and received signals respectively. These downconverted signals are sent to Control and Data Acquisition module 3 fordata acquisition, digitization, display and recording.

The recorded signals are processed to compress the signals using the FFTmethod of the invention described here in and in Example 1 and Example 2to determine the round trip time delay to the tag. Measurements weretaken for various fixed distances D. Calculations were performed usingMicrosoft's Excel® spreadsheet program. Calculations could be made byvarious methods including microprocessor, DSP, or equivalent.

The approach of FIG. 7 results in consistent transmitted and receivedsignals although the monostatic radar system has several limitations.Leakage of RF signals in Circulator 16 and unmodulated reflections fromradar antenna 18 produce signals that are much higher than the modulatedsignals received from the tag 20, resulting in a diminished sensitivity.Sensitivity is improved by the bistatic system of FIG. 8 that eliminatescirculator 16 of FIG. 7. Signals from the tag are received by antenna21, amplified by amplifier 22, and down-converted by mixer 8. Dataacquisition and processing may be accomplished as with the system ofFIG. 7. The system of FIG. 8 decreases the signal levels of unmodulatedsignals not originating with the tag and allow amplification of thesmall received signals without resulting in signal compression. Noise inthe system is also due to the quantization of time and amplitude by theanalog to digital converters in the Control and Data Acquisition module3. The bistatic architecture of FIG. 8 also helps decrease the unwantedeffects in the A/D converters without resulting to converters operatingat faster speeds and with higher resolution.

FIG. 9 shows the signals resulting for a vehicle 14 feet from the RFIDand radar antennas. The bistatic differential pulse compression radartransmitted a chirp signal beginning at 902 MHz and ending at 928 MHzwith a duration of 2 microseconds for each modulation state of the tag.A down-converted transmitted signal is shown in FIG. 9 a. Thedown-converted received reflected signals are shown in FIG. 9 b for thetag in modulation state A, and in FIG. 9 c for the tag in modulationstate B. The Fast Fourier Transform method of implementing a matchedfilter produces the compressed signals shown in FIG. 9 d for the tag inmodulation state A, and in FIG. 9 e for the tag in modulation state B.The differential compressed signal from the tag is shown in FIG. 9 fwhich is the subtraction of the signals of FIGS. 9 b and 9 c, thusremoving constant reflections leaving the signal originating from themodulation produced by the tag. The power in the compressed signal fromthe tag is shown in FIG. 9 g. The peak of the signal of FIG. 9 g occursat 185.5 nanoseconds. It is convenient to use the power of thecompressed signal to identify the peak of the correlation since it isnot know apriori which of the modulation states of the tag will producehigher signals (eg. should A-B or B-A be used? Either will eliminate theconstant background leaving a signal produced by tag modulation).Alternately, the amplitude of the correlation signal may be used.

The long delay time is due to the delay resulting from the cablesconnecting the roadside radar system to the radar antennas. The systemcan be calibrated noting the delay time for a tag at a particularlocation. Then the delay for a tag at an unknown location can becompared with that for the known location. The difference in timing isthen used to calculate the location of the tag with respect to the taglocation corresponding to the known delay.

This example uses a chirp signal with a linear sweep and constantamplitude. The methods may also be used for other modulations includingbut not limited by CDMA, non-linear chirp, AM, FM, phase modulation,Barker codes, and other modulation codes.

Active Transmitter Tag RFID Systems

Active RFID tags transmit data to readers by generating and modulatingRF signals sent to a reader. Various modulation waveforms and codes maybe used. A common modulation is amplitude on/off keying using Manchestercoding. The reader recovers the clock frequency and decodes theManchester modulation to recover the data sent by the tag. The followingapproach may also be used with other forms of modulation and coding.

Tags are relatively simple and inexpensive devices. As such, an on/offkeying may be accomplished by switching a source of RF signal within atag on and off to a tag antenna. This switching changes the loadimpedance connected to the antenna, and thus may change the backscattercross section of the tag antenna as the RF source is switched on andoff. Thus, the methods of this invention may be used to measure thedistance to RFID systems using active (transmitter) tags. Thesensitivity of the radar system will be good because of the ability touse a super heterodyne receiver and pulse compression techniques. Thus,active transmitter tags may be designed to enhance the modulatedbackscatter cross section of the tag antenna, or the residual modulatedbackscatter cross section may used, either method may use the presentinvention to measure the distance to an active RFID tag in the presenceof other RF signals, clutter, multipath and noise.

Those skilled in the art will recognize other detailed designs andmethods that can be developed employing the teachings of the presentinvention. The examples provided here are illustrative and do not limitthe scope of the invention, which is defined by the attached claims. Forexample, disclosure with respect to waveforms for encoding orrepresenting data can apply equally well to the inverses of thewaveforms used as examples.

1. A method for measuring range to an RFID tag having a plurality ofmodulation states including situations containing high clutter andmulti-path signals, comprising the steps of: transmitting a first radarsignal coordinated with a first modulation state of the tag;transmitting a second radar signal coordinated with a second modulationstate of the tag; receiving reflected radar signals for said first andsecond modulation states of the tag; processing each of receivedreflected radar signals using a fast Fourier transform (FFT) to producecomplex signal spectrums; forming a complex conjugate of the FFT of oneof said transmitted radar signals; multiplying said complex conjugate ofthe transmit signal with the FFT of each received signal individually tocreate first and second products; producing a correlation for eachreceived signal individually by performing an inverse FFT of each ofsaid first and second products; subtracting the correlation obtainedwith the tag modulation in said first state from the correlation withthe tag modulation in said second state to produce a signal having apeak; determining round trip time delay to the RFID tag from the time ofsaid peak.